The increased consumption of fossil fuels, in particular natural gas and oil, has revealed that these fuels are a limited resource. Thus, utilisation of less noble fossil fuels like heavy fuel oil and coal has become more pronounced in particular in countries which rely on import of natural gas or oil.
Coal is a major energy source for electric power generation and electricity can be produced with a high efficiency from coal by gasification. Coal can also be used as a feedstock for several chemical processes such as methanol, dimethyl ether and ammonia synthesis or for power generation by fuel cells. Coal contains many harmful impurities, for instance sulphur and halogens, and these impurities have to be removed to satisfy environmental regulations or to a level required by processes downstream gasification. Processes for cleaning coal gases resulting in very low emissions of harmful gases such as sulphur dioxide and other gases, for instance nitrogen oxides, are therefore being studied and developed worldwide.
Topical Report Number 19, Tampa Electric Integrated Gasification Combined-Cycle Project—An Update (July 2000), published by The Office of Fossil Energy, the U.S. Department of Energy on the following internet address:
http://www.fe.doe.gov/programs/powersystems/cleancoal/publications/
discloses a coal gasification process used to fuel a gas turbine generator, which has an exhaust that is integrated with a heat recovery steam generator and a steam turbine generator for power generation. Coal is partially oxidised in a gasifier with 95% pure oxygen to produce raw synthesis gas which is then cooled and washed with water for particulate removal followed by washing with water to convert carbonyl sulphide (COS)to hydrogen sulphide(H2S). After additional cooling, the raw synthesis gas is sent to a conventional acid gas removal unit, where H2S is absorbed by scrubbing with an amine solvent. H2S is removed from the amine by steam stripping and sent to the sulphuric acid plant.
Technical Progress Report Gasification Test Run GCT2, DOE Cooperative Agreement Number DE-FC21-90MC25140 (Apr. 10-27, 2000) available on the following internet address:
http://psdf.southernco.com/tech_progress_reports.html
discloses a process for cleaning coal gas in which a fluidized bed reactor operating as either a combustor or a gasifier is fed with coal particles and a sorbent material such as limestone or dolomitic sorbents. The sorbent material captures any sulphur present and converts it to calcium sulphide (CaS). The gas stream leaving the gasifier is passed through a barrier filter to filter out dust from the reactor. The gas stream is then sent to a thermal oxidiser to oxidise all reduced sulphur compounds (H2S, COS, CS2) and reduced nitrogen compounds (NH3, HCN). Gasification solids are processed in a sulfator to oxidise CaS to calcium sulphate (CaSO4) suitable for commercial use or disposal. Alternatively, the gasification solids could be burned to recover the heating value of the residual carbon content.
U.S. Pat. No. 5,169,612, incorporated herein by reference, discloses a process whereby sulphides are removed from gas streams by contacting the gas stream with a solid absorbent containing tin oxides and a stabilising component, and thereafter regenerating the absorbent by contacting it with steam. The solid absorbent has an active absorption component of tin, tin oxides or mixtures thereof, and a carrier material which can be a refractory material such as clays, silicates, alumina and titania. In addition a stabilising component may be present. The formation of sulphates as by-products are avoided.
U.S. Pat. No. 4,769,045 discloses a process whereby sulphur-containing gas from a gasifier is passed through an external bed of a regenerable solid absorbent of preferably zinc ferrite for desulphurising the gas. The absorbent is regenerated by passing a mixture of steam and air or oxygen through the bed for converting absorbed hydrogen sulphide to sulphur dioxide. The resulting gas mixture of sulphur dioxide and steam is sent to the gasifier for conversion by a calcium compound into a stable form of sulphur such as calcium sulphate.
Due to the low pressure of the obtained stream of produced sulphur dioxide and steam, further compression is required before transfer to the gasifier, since the gasifier operates at a higher pressure.
WO patent application No. 96/30465 discloses a process similar to the process disclosed in U.S. Pat. No. 4,769,045. Sulphur dioxide is formed during regeneration of the sorbent. The regeneration offgas is passed into an ejector to increase its pressure to at least a level equal to the pressure of the gasifier. Air is used as motive gas in the ejector.
U.S. Pat. No. 6,428,685 discloses a process for removing sulphur from a stream of cracked-gasoline or diesel fuel. The process includes contacting the stream with a sorbent composition of zinc oxide, silica, a promoter metal, alumina and a calcium compound. The sorbent is regenerated by desulphurising the sorbent using an oxygen-containing gas such as air, and thereafter reducing the promoter metal in the sorbent to metal with a reducing agent.
A hot gas cleaning process is disclosed by P.E. Højlund Nielsen et al. in Hot Gas Cleaning of Coal Gases by Sequential Absorption, Proceedings of the Twelfth Annual International Pittsburgh Coal Conference, (Sep. 11-15, 1995) pages 1074-1079, incorporated herein by reference. A process for deep desulphurisation of coal gases using a “sandwich” absorbent composed of two different absorbents based on tin dioxide (SnO2) and zinc oxide (ZnO), respectively. Tin dioxide is used for bulk desulphurisation, while zinc oxide is used in a polishing mode. This allows the use of steam with a small addition of oxygen as a regeneration agent. The regeneration gas contains hydrogen sulphide, allowing the recovery of sulphur as elemental sulphur by for instance a Claus unit or the conversion of H2S to concentrated sulphuric acid by a wet sulphuric acid unit.
Sigurdardottir, I.D. et al. have tested the use of a sorbent sandwich based on tin oxide and zinc oxide sorbents as a desulphurisation agent using coal gases and regeneration of the sorbent sandwich using steam containing oxygen. This is further described in the proceedings of 1995 Annual Meeting Session No. 259, American Institute of Chemical Engineers, Miami Beach, Fla., Nov. 12-17, 1995, incorporated herein by reference.
The disadvantage of the above mentioned processes is that the sorbent regeneration loop results in steam containing hydrogen and hydrogen sulphide and recovery of sulphur as elemental sulphur is costly. There is therefore a need for a simpler process.
It has now surprisingly been found that by converting sulphur to hydrogen sulphide during regeneration and by returning the hydrogen sulphide-containing regeneration steam directly to the gasifier all sulphur is removed as calcium sulphide.
Advantageously, additional pressurizing of the hydrogen sulphide-containing regeneration steam in order to raise the pressure to a level equivalent to the pressure in the gasifier is not necessary.
It is an objective of the invention to provide a simple process with a high thermal efficiency for cleaning gases from gasification units.
It is also an object of the invention to provide a cleaned shifted gas from a gasification unit suitable for use in the preparation of for instance ammonia or methanol.